In my U.S. Pat. No. 3,624,532 (also see U.S. Pat. Nos. 1,834,002 and 3,525,052) a feedback amplifier is described wherein a component of the amplifier output signal is compared with a component of the amplifier input signal and a difference, or error signal produced. The latter, which represents the distortion introduced by the principal amplifier, is amplified in a subsidiary error amplifier and then fed back to the input end of the principal amplifier so as to minimize the distortion in the amplifier output signal.
As noted in my above-cited patent, it is an advantage of a differential feedback amplifier that only the error is fed back and, hence, only the error is degenerated by the feedback process. It is a second advantage of such an amplifier that the dynamic range of the error amplifier is very much less than the dynamic range of the principal amplifier since the former is only required to handle the error signal and, hence, will typically have a much broader bandwidth than the principal amplifier. As a result, the overall frequency sensitivity of the feedback loop of a differential feedback amplifier is significantly less than that of a conventional feedback amplifier having the same net gain. Consequently, the stabilized bandwidth of the former is much greater.
The above-described advantages are readily realized but within limits. For example, at the higher frequencies, the typical rolloff in the principal amplifier gain characteristic introduces added time delay, and causes an increase in the magnitude of the error signal that must be handled by the error amplifier. Thus, any attempt to expand the operating frequency so as to include these higher frequencies will require a much larger error amplifier of reduced bandwidth. The net result of the combination of additional time delay through the principal amplifier, and the added delay through a larger error amplifier is to cause a reduction in the overall stability of the amplifier.
It is, accordingly, the broad object of the present invention to extend the stable operating range of a differential feedback amplifier.